The invention relates to a method of improving the resolution of a medical nuclear image of a body volume by combining it with an ultrasound image of the same body volume. The invention also relates to a device which includes a memory and a data processing unit and is suitable for carrying out such a method.
Medical nuclear methods have become very prominent in medical diagnostics, because they enable the visualization of body functions via observation of the absorption of radioactive marker substances. Hereinafter SPECT (Single Photon Emission Computed Tomography) and PET (Positron Emission Tomography) will be considered by way of example. These methods yield three-dimensional representations of the metabolic activity of the heart muscle, but these images have a comparatively poor spatial resolution so that an exact diagnosis of the heart function is impeded in many cases.
On the other hand, it is known that sectional images of the heart can be acquired with a favorable resolution by means of ultrasound methods. In the case of ultrasound operation in the so-called B mode, a sound wave having a pronounced directional characteristic is applied to the tissue. The echo signal reflected by the tissue is demodulated, logarithmated and stored. Subsequently, a new sound wave is applied along a different trajectory which, like all previous sound wave trajectories, however, is situated in the plane to be imaged. After the plane has been sufficiently densely scanned in this manner, the demodulated and logarithmated echoes are encoded as grey values and arranged in the image plane in such a manner that an image of the objects in the scanned plane is obtained which is faithful in respect of length and angle. The described scanning of the plane can be performed by means of a fan-shaped, parallel or other configuration of the sound wave trajectories. An ultrasound image of a volume can be obtained by selecting the trajectories of the sound waves to be such that they are not situated in one plane but scan a volume (for example, conically or by parallels). It is a common aspect of the ultrasound images that they represent only the anatomy and not the function of the heart muscles.
Furthermore, from literature (J. P. Caravel et al., Fusion d'images anatomique (échographie) et fonctionnelles (tomoscintigraphie) rénales, MÉDECINE NUCLÉAIRE, 1995, Elsevier) a method is known for the imaging of the kidneys of a patient; according to this method first a three-dimensional scintigraphic image and subsequently a two-dimensional ultrasound image are formed of the kidneys. In that case the ultrasound apparatus is provided with position markers (light-emitting diodes) which are detected by a camera system so as to be related to a stationary reference system. The position in space of the ultrasound apparatus or the ultrasound image as well as the position of the scintigraphically imaged body volume are then determined by way of appropriate calculations; subsequently, a sectional image corresponding to the ultrasound image can be calculated from the three-dimensional scintigraphic image so as to be superposed on the ultrasound image. Such a method has the drawback, however, that a large amount of work is required for the optical sensing of the position of the ultrasound apparatus and the scintigraph. Furthermore, the precision of the method is limited by the precision of the optical position sensing. Moreover, a systematic error effect is introduced by the fact that a motion of the patient, for example, due to respiration, may take place between the image acquisitions, so that imaging may be based on incorrect assumptions in respect of position.